Make the streusel by cutting the cold butter into the flour, brown sugar, and cinnamon until crumbly, then refrigerate.
Cream the softened butter, granulated sugar, and brown sugar for the dough, then beat in the egg and vanilla.
Mix in the dry ingredients (flour, baking powder, cinnamon, salt) just until a soft dough forms.
Scoop the dough into balls, flatten them slightly on a baking sheet, and press a generous mound of chilled streusel onto each.
Bake at 350 degrees Fahrenheit for 11 to 13 minutes, cool completely, and drizzle with the mixed powdered sugar glaze.